Question

I often see code similar to this:

return [(var, val) for val in self.domains[var]
                    if self.nconflicts(var, val, assignment) == 0]

and I’m like DAMN that’s sexy. But then I try to drop it sometimes and I get syntax errors. Are there any particular rules for this nice form of code writing that reverses the typical placement of for and if statements?

Answer 1

They’re called list comprehensions. The basic syntax is (I’m using parens to group my words, not as part of the syntax):

[(an expression involving x) for x in someList if (some condition)]

If the condition evaluates to true, the resulting list includes the (expression involving x). So, for example, the following list comprehension uses this to only include strings in the resulting list.

>>> myList = [1,"hello",5.4,"world"]
>>> [elem for elem in myList if type(elem)==str]
['hello', 'world']

Note that the if part is optional, and the expression involving x can be as simple as just x (often used when you are just filtering out elements from another list).

In fact, the expression involving x doesn’t really have to have x in it at all. For example, if for some reason you wanted a list of 0‘s as long as your name you could do this:

>>> [0 for letter in "Matthew"]
[0, 0, 0, 0, 0, 0, 0]

For when you don’t need the list to stick around after you make it, use generator expressions instead. (Generator expressions and list comprehensions have the same syntax.)